Chemical Components of the Cell

The most important molecule in the cell is water; the life of all organisms depends on its special properties.

Water

Unquestionably, water is the most important molecule in the world. Water makes up over 70% of all living organisms by weight. Most of the compounds of living beings are in solution inside cells. The importance of water to life on earth in large part depends on its unusual properties.

Bonding Properties of Water

Hydrogen bonding plays a very important role in giving water the properties which are required for life (figure 2.8a). Since water is a polar molecule, the positive H portion of the molecule is attracted to the negative O portion of other water molecules, thereby creating hydrogen bonds. The extent of hydrogen bond formation between water molecules depends on the temperature. At room temperature, when water is in a liquid state, the weak bonds continually form and break. As the temperature is lowered, the breakage and formation decreases, and in ice, a crystalline structure is formed. Each water molecule bonds to four other molecules to form a rigid lattice structure (figure 2.8b). When ice melts, the water molecules can move closer together. Consequently, liquid water is more dense than ice, which explains why ice floats. This explains how fish and

(a) Liquid Water

(a) Liquid Water

Figure 2.8 Water (a) In liquid water, each H2O molecule hydrogen bonds to one or more H2O molecules.These bonds continually break and re-form. (b) In ice, each H2O molecule is hydrogen bonded to four other H2O molecules, forming a rigid crystalline structure.The bonds do not break continuously.

Nester-Anderson-Roberts: I I. Life and Death of I 2. The Molecules of Life I © The McGraw-Hill

Microbiology, A Human Microorganisms Companies, 2003

Perspective, Fourth Edition bacteria can apparently live in frozen bodies of water. They actually live in the water, which remains liquid below the ice.

The polar nature of water also accounts for its ability to dissolve a large number of compounds. Water has been referred to as the universal solvent of life because it dissolves so many compounds. To dissolve in water, compounds must contain atoms with positive or negative charges. When placed in water, they ionize or split into their component charged atoms. For example, NaCl dissolves in water to form Na+ ions, and Cl: ions. In solution, ions such as Na+ and Cl: tend to be surrounded by water molecules in such a way that the OH: of HOH forms weak bonds with Na+, and the H+ forms weak bonds with Cl: (figure 2.9). The Na+ and the Cl: cannot come together, and this accounts for the solubility of NaCl in water.

Water containing dissolved substances freezes at a lower temperature than pure water. Because the water molecules are hydrogen bonded to the dissolved ions, a much lower temperature is required for the water molecules to assume the rigid lattice structure of ice. Thus, in nature, most water does not freeze unless the temperature drops below 0°C. Consequently, microorganisms can usually grow in liquids at 0°C, the freezing temperature of pure water.

An important property of every aqueous solution is its degree of acidity. This property is measured as the pH of the solution (an abbreviation for potential Hydrogen), defined as the concentration of H+ in moles per liter. pH is measured on a logarithmic scale of 0 to 14 in which the lower the number, the more acid the solution. The acidity of a solution is based on several properties of water. Water has a slight tendency to split (ionize) into hydrogen ions H+ (protons), which are acidic, and OH: ions (hydroxyl), which are basic or alkaline. In

Figure 2.9 Salt (NaCl) Dissolving in Water In the absence of water, the salt is highly structured because of ionic bond formation between Na+ and Cl: ions. In water, the Na+ and Cl: are separated by H2O molecules.The Na+ hydrogen bonds to the slightly negatively charged O: and the Cl: hydrogen bonds to the slightly positively charged H+ portion of the water molecules.

Nacl Water

Figure 2.9 Salt (NaCl) Dissolving in Water In the absence of water, the salt is highly structured because of ionic bond formation between Na+ and Cl: ions. In water, the Na+ and Cl: are separated by H2O molecules.The Na+ hydrogen bonds to the slightly negatively charged O: and the Cl: hydrogen bonds to the slightly positively charged H+ portion of the water molecules.

2.3 Chemical Components of the Cell 23

pure water, only one molecule in 10 million molecules (1 in 107) ionizes spontaneously:

As this reaction indicates, when water splits into its component parts, the number of H+ and OH: ions is equal, and in pure water, the concentration of each is 10:7 molar (10:7 M). The product of the concentration of H+ and OH: must always be 10:14 M (10:7 x 10:7). (Recall that exponents are added when numbers are multiplied.) Thus, if H+ ions are added so that the concentration of H+ increases 10-fold to 10-6 M, then the concentration of OH: must decrease by a factor of 10 (to 10-8 M).

The pH scale, which is logarithmic, simplifies the relationship between the concentration of H+ and OH: ions (figure 2.10). The scale ranges from zero to fourteen because the concentrations of H+ and OH: ions varies within these limits. When the concentration of H+ and OH: are equal, the pH of the solution is 7 and is neutral. Most bacteria can live within only a narrow pH range, near neutrality. Some, however, can live under very acidic conditions (acidophiles) and a few under alkaline conditions (alkalophiles). ■ acidophiles, p. 89 ■ alkalophiles p. 89

OH" H

concentration concentration

pH
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